Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonance frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
MRI systems may utilize a superconducting magnet to generate a main magnetic field, B0. A superconducting magnet includes superconducting coils that are enclosed in a cryogenic environment within a cryostat (or magnet vessel) designed to maintain the temperature of the superconducting coils below an appropriate critical temperature so that the coils are in a superconducting state with zero resistance. For example, the windings of the superconducting magnet may be immersed in a bath or vessel of liquid helium to maintain the temperature below the critical temperature for superconducting operation. During installation and start-up of an MRI system, the superconducting magnet is energized (or ramped) by introducing electrical current to generate the appropriate main magnetic field strength. Typically, a large power supply (e.g., 1000 Amps) may be used to provide current to the superconducting magnet coils.
The superconducting magnet may also require additional energy during the operating life of the MRI system after installation. MRI systems require a uniform main magnetic field in the imaging volume, however, the main magnetic field may drift or decay over time after installation due to various factors such as imperfections in the magnet. Change or drift in the main magnetic field can adversely affect the performance of the MRI system including data acquisition and reconstruction of an MR image. Accordingly, energy may need to be provided to the superconducting magnet during maintenance to return (e.g., increase) the main magnetic field to the appropriate strength. As mentioned, a large power supply is typically used to provide energy to the superconducting magnet.
Conventional methods of energizing superconducting magnets, however, have several disadvantages. The large power supplies can be heavy and expensive. In addition, the power supply may utilize large, high rated current leads that are connected to the cryostat and designed to handle the high electrical current required by the main coils of the superconducting magnet. The connections to the main coils in the cryostat can lead to loss of liquid helium which is expensive to replace.
It would be desirable to provide a system and method for energizing a superconducting magnet that reduces or eliminates liquid helium loss and reduces the cost of installation, operating and servicing an MRI system.